1. Technical Field
The present invention relates to a semiconductor light emitting device having a light emitting diode (LED) or any other semiconductor light emitting element mounted on a wiring substrate.
2. Description of the Related Art
A semiconductor light emitting device having an LED mounted on a wiring substrate has been used in an illuminator or lighting fixture, a backlight, an industrial apparatus, and other apparatus. In recent years, since the amount of heat generated in a semiconductor light emitting device itself increases as the intensity of the light from the semiconductor light emitting device increases, there is an increasing necessity to quickly dissipate the heat generated in the semiconductor light emitting device.
To increase heat dissipation performance, there has been a known technique for forming a wiring substrate or wiring board on which an LED is mounted with a plurality of ceramic layers and providing heat dissipation vias through the wiring substrate formed of the plurality of ceramic layers. The thus configured wiring substrate can dissipate a larger amount of heat because the heat dissipation vias are made of silver or copper having heat conductivity higher than that of the ceramic of which the wiring substrate is made. Japanese Patent Application Laid-Open Publication No. 2008-172113 (hereinafter also referred to as Patent Document 1), for example, describes a wiring substrate based on the technique described above.
Since silver used for the heat dissipation vias is suitable for a material that reflects light emitted from an LED (that is, having high reflectance), a technique for providing a reflective layer made of silver on a surface of a wiring substrate has also been known. On the other hand, since silver readily reacts with sulfur, silver sulfide tends to be produced in the portion of the wiring substrate that is exposed to the atmosphere. Since the reflectance of silver sulfide is lower than that of silver, the intensity of the light from the semiconductor light emitting device disadvantageously decreases with time. Further, the chromaticity of the light and external appearance of the semiconductor light emitting device change with time when the silver is sulfurized. Further, when the reflective layer made of silver is irradiated with the light emitted from the LED, part of the silver in the reflective layer is activated and diffused in a light transmissive resin, resulting in a problem similar to that occurs due to the sulfurization of silver described above.
To solve the problems described above, there has been a known technique for coating or covering the reflective layer with a light transmissive material to prevent the silver from being sulfurized and the activated silver from floating in the light transmissive resin. Japanese Patent Application Laid-Open No. 2010-34487 (hereinafter also referred to as Patent Document 2), for example, describes a wiring substrate based on the technique described above. In the wiring substrate described in Patent Document 2, a reflective layer made of silver is coated with a glass layer.